This invention relates to an aqueous coating composition useful for making a gas diffusion layer for an electrochemical cell such as a fuel cell, which layer is advantageously free of surfactant residue. The coating composition comprises carbon particles, one or more highly fluorinated polymers, and one or more amine oxide surfactants.
U.S. Pat. No. 6,127,059 describes the use of Triton(trademark) X100, an alcohol alkoxylate, as a surfactant when coating a gas diffusion layer for use in an electrochemical cell with a fluororesin containing carbon black.
Briefly, the present invention provides an aqueous coating composition comprising carbon particles, one or more highly fluorinated polymers and one or more amine oxide surfactants, typically an amine oxide surfactant according to formula I: 
where n is 9 to 19.
In another aspect, the present invention provides a method of making a gas diffusion layer for an electrochemical cell comprising the steps of: a) coating a electrically conductive porous substrate with the aqueous coating composition according to the present invention, and b) heating the electrically conductive porous substrate to a temperature sufficient to decompose the amine oxide surfactants.
In another aspect, the present invention provides a gas diffusion layer for an electrochemical cell made according to the method of the present invention which typically contains no surfactant.
What has not been described in the art, and is provided by the present invention, is a coating composition and method of using same which contains a decomposing amine oxide surfactant and can therefore provide a surfactant-free gas diffusion layer.
In this application:
xe2x80x9cvehiclexe2x80x9d means a fluid which carries the particulate in a dispersion, which typically includes water or an alcohol;
xe2x80x9chighly fluorinatedxe2x80x9d means containing fluorine in an amount of 40 wt % or more, but typically 50 wt % or more, and more typically 60 wt % or more; and
xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide a coating composition and method of using same which contains an effective surfactant which is ephemeral in use, and therefore capable of providing a surfactant-free gas diffusion layer.